1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor substrate, and more particularly to a semiconductor device having mounted therein electric field reducing means for increasing dielectric strength.
2. Description of the Related Art
In an integrated circuit formed by mounting various circuit elements, such as transistors, in a semiconductor substrate, a first impurity region is formed, in the surface of the semiconductor substrate, as a functional element that has a conductivity type opposite to the conductivity type of the semiconductor substrate. When a backward voltage is applied between the first impurity region and the semiconductor substrate through a current path extending on the oxide film covering the semiconductor substrate, a depletion layer expands from the impurity region along the surface of the semiconductor substrate below the current path according to the voltage value.
When the depletion layer reaches the surrounding circuit area, it exerts an adverse effect on the electrical characteristics of the circuit area. To curb the expansion of the depletion layer toward the surrounding circuit, there is a technique to provide as a channel stopper in the semiconductor substrate a second impurity region which is of the same conductivity type as the semiconductor substrate and has a higher impurity density than that of the substrate.
However, the channel stopper is not enough because when a relatively low voltage is applied, the depletion layer would reach the channel stopper and as the voltage is increased, a breakdown occurs in the depletion at a lower voltage than the destructive voltage at the junction plane between the first impurity region and the semiconductor substrate.
Therefore, as disclosed in JP-A-11-204632 (Japanese Patent Application No. 10-17848), the present inventors have proposed an electric field reducing technique to arrange electrodes mutually spaced apart from each other along the current path in the oxide film under the current path, and to connect the potential of the semiconductor substrate to the electrodes.
According to the field reducing technique, the expansion of the depletion layer is controlled by degrees by the electrodes as the applied voltage is increased, or in other words, by allowing the depletion layer to expand step by step as the voltage is increased, it is possible to consider a balance between the breakdown voltage in the depletion layer and the destructive voltage at the junction, which makes it possible to improve the general dielectric property of the semiconductor device.
However, the dielectric strength improving technique such as this requires that the electrodes supplied with a specified potential be arranged spaced apart from each other in the expanding direction of the current path and that a specified spacing be provided between the electrode. Therefore, such a technique is not advantageous for dimensional reductions to obtain semiconductor devices in reduced sizes where a large number of electrodes are required.
The object of the present invention is to provide a semiconductor device that can be reduced in size without sacrificing the dielectric property.
The present invention has been made with attention directed to the fact in the above-mentioned field reducing technique that the expansion of the depletion layer is controlled by degrees such that a balance between the breakdown voltage in the depletion layer and the destructive voltage at the junction can be maintained by combining two opposite functions: the restraint of the depletion layer expansion by the electrodes and the prolongation of the depletion layer at the spacing areas between the electrodes in the insulating film in which the electrodes are buried. Basically, the feature of the present invention is that means for promoting the expansion of the depletion layer more than the insulating film does is arranged between the electrodes to thereby substantially make narrower the spacing between the electrodes without disturbing the balance mentioned above, with the result that a smaller-size semiconductor device can be produced without reducing the dielectric strength.
According to the present invention, there is provided a semiconductor device which comprises a semiconductor substrate of one conductivity type, p-type or n-type; a first impurity region of the other conductivity type, p-type or n-type, having applied between itself and the semiconductor substrate a backward voltage through an current path extending on an electrically insulating film providing on the surface of the semiconductor substrate; a second impurity region of the same conductivity type as the semiconductor substrate and with a higher impurity density than the impurity density of the semiconductor substrate, the second impurity region being formed in the surface of the semiconductor substrate spaced from the first impurity region, to restrain the depletion layer from expanding the first impurity region below and along the current path; and a suppressor electrode layer comprising a plurality of thin portions arranged in the electrically insulating film and mutually spaced apart from each other in the direction of the current path and placed at substantially the same potential as the semiconductor substrate, to restrain the depletion layer from expanding toward the second impurity region.
According to the present invention, expansion promotion means for promoting the expansion of the depletion toward the second impurity region may be arranged between the narrow portions of the suppressor electrode layer.
The above-mentioned expansion promotion means for more efficiently promoting the expansion of the depletion layer than the electrically insulating film, which has buried therein the narrow portions of the suppressor electrode layer, makes it possible to cause the spacing of the respective narrow portions to become narrower and also cause the distance between the two impurity regions to become shorter without incurring loss of the field reduction effect of the field reducing means, which includes the expansion promotion means and the suppressor electrode layer. Therefore, the semiconductor device can be reduced in size without decreasing its dielectric property.
The expansion promotion means may be formed by the thin portion of the electrically insulating film defined by the surface of the semiconductor substrate and the protrusions, protruding toward the substrate between the narrow portions, of the current path extending on the insulating film on the semiconductor substrate.
The electrically insulating film at its thin portions promotes the expansion of the depletion layer by the electrostatic effect by the potential applied to the current path on the insulating film.
The expansion promotion means may be formed by expansion promotion electrodes arranged in the areas corresponding to the spaces between the narrow portions of the suppressor electrode layer.
The expansion promotion electrodes may be arranged spaced from the narrow portion of the suppressor electrode layer on the same plane as the suppressor electrode layer. As an alternative to this arrangement, the expansion promotion electrodes may be arranged in the electrically insulating film in the areas on a plane between the suppressor electrode layer and the semiconductor substrate surface, which correspond to the spaces between the narrow portions.
The expansion promotion electrodes may be placed in electrically floating state or at a potential substantially equal to the potential of the first impurity region. Either way, the expansion of the depletion layer is promoted more than the electrically insulating film that fills up the spaces between the narrow portions of the suppressor electrode layer. To increase the depletion layer expansion effect, it is desirable that the expansion promotion electrodes should be supplied with a potential substantially equal to the potential of the first impurity region.
The expansion promotion means may be formed by a dielectric material that has a larger dielectric constant than that of the electrically insulating film. Local increases in dielectric constant of the electrically insulating film, which act as substantial decreases in the thickness of that insulating film, offer the same expansion reducing effects as local decreasing of thickness of that insulating film.
Instead of providing the expansion promotion means, the narrow portions of the suppressor electrode layer may be arranged in a circumferential direction of the first impurity region.
The current path arranged on the electrically insulating film, which has the suppressor electrode layer buried therein, includes a deformed portion, as a part of the current path, arranged in the direction in which the narrow portions are arranged.
In this case, a required number of narrow portions are arranged not in a straight line from the first impurity region to the second impurity region as the channel stopper, but in a circumferential direction of the first impurity region. Therefore, even when a large number of narrow portions are required, it is possible to arrange a large number of narrow portions of the suppressor electrode layer without incurring any dimensional increase in the distance between the two impurity regions.
Further, even by making a contrivance to the slits between the narrow portions of the suppressor electrode layer, the semiconductor device can be reduced in size without decreasing its dielectric strength.
More specifically, the slits between the narrow portions of the suppressor electrode layer can be added with side extensions extending beyond both side edges of the current path, which stretches from the first impurity region, and inclined at an acute angle toward the extending direction of the current path. Because the slits, which are filled with the electrically insulating film, extend beyond the side edges of the current path while the side extensions are inclined toward the extending direction of the depletion layer, the side extensions guide the depletion layer along their extending directions, so that it becomes possible to effectively prevent a breakdown from occurring in the depletion layer and reduce the width of the respective slits, in other words, the spacing between the narrow portions.